The present invention relates to a photonic crystal, a method of fabricating the same, an optical module, and an optical system. More specifically, the present invention relates to a photonic crystal having a stepped ridge waveguide on the side, surfaces of which steps of about 0.5 μm are symmetrically formed, and capable of being easily and reliably fabricated, to a method of fabricating the same, to an optical module, and to an optical system.
In a structural layer called a “photonic crystal” in which two types of optical media having different refractive indices are periodically arrayed on the wavelength size of light, the relationship between the wave number of light and the frequency, i.e., the photon energy shows a band structure owing to a periodic refractive index change, like the energy of electrons in a semiconductor shows a band structure in a periodic potential.
Photonic crystals having one-dimensional periodic distributions are known as dielectric multilayered films. However, as the periodic distribution becomes two-dimensional and three-dimensional, a photonic crystal exhibits specific optical properties. For example, in a photonic crystal it is possible to produce a wavelength region called a “photonic bandgap” in which light does not propagate in any direction (E. Yablonovitch, Phys. Rev. Lett. 58(20), 2059(1987)). Also, a photonic crystal shows very large optical anisotropy or dispersion. That is, the optical characteristics of a photonic crystal are very distinctive.
When “irregularity” such as a defect with respect to a photonic crystal is introduced to a photonic crystal, the photons are permitted to exist only in that portion. That is, an optical waveguide having high selectivity can be realized. A greater advantage when a photonic crystal is used in an optical waveguide is that a light wave can be sharply bent with no loss in an optical waveguide, a “bend” of the waveguide must be formed with smooth and gradual change since light is scattered and lost by radiation from a steep curve. By contrast, in a photonic crystal light is interrupted by a photonic bandgap except in a waveguide. This can realize a sudden waveguide bent at a right angle. Also, since no smooth curve is necessary, an optical device having a waveguide can be greatly miniaturized.
As described above, a photonic crystal, particularly a three-dimensional photonic crystal has useful features.
A three-dimensional photonic crystal can be formed by arranging periodic structures such as semiconductor/air diffraction gratings in parallel crosses. This structure is called a “wood pile”. In a wood pile photonic crystal, it is important to shift, by a half period, the phases of every other, parallel diffraction gratings. A semiconductor/air periodic structure is desirable because the refractive index of one medium of the periodic structure must be twice that of the other (medium) of the periodic structure or larger in order to effectively achieve the properties of a photonic crystal.
Noda reported that a photonic crystal was implemented by the wafer adhesion (fusion) technology (e.g., Journal of Electronic Information Communication Society, 1999, March, pp. 232–241).
FIGS. 12A to 12D are schematic views conceptually showing the formation steps of the photonic crystal reported by Noda. First, as shown in FIG. 12A, a wafer is prepared, in which a diffraction grating 20 having a semiconductor/air stripe structure is formed on the surface of a semiconductor substrate 30. Another wafer in which a diffraction grating 21 is similarly formed on a substrate 31 is also prepared. As shown in FIG. 12B, these two wafers are aligned and fused in parallel crosses on their diffraction grating surfaces.
As shown in FIG. 12C, the substrate 30 is removed by a selective etchant. After that, a wafer similar to that shown in FIG. 12A is rotated 90° and fused such that its diffraction grating surface opposes the other. The substrate of this wafer is removed by a selective etchant. By repeating this step, a structure as shown in FIG. 12D is obtained. In this structure, the phases of every other, parallel stripe diffraction gratings are shifted a half period.
Unfortunately, the above-mentioned method has the following drawbacks.
(1) A hard-to-fuse photonic crystal is difficult to implement.
(2) The substrate removal step is cumbersome, and this substrate removal is also a waste of resources.
(3) The surface of the photonic crystal is uneven, so flat crystal growth is difficult to perform on that surface.
(4) A photonic crystal is not easily formed only on a part of a wafer.
It is one object of the present invention to provide a novel photonic crystal which overcomes the above-mentioned drawbacks, and a method of fabricating the same.
It is another object of the present invention to provide a novel optical device using a photonic crystal. The problems of optical devices relevant to the present invention will be described below by taking a semiconductor laser as an example.
Cleavage surfaces must be formed in conventional semiconductor lasers. This will be explained with reference to a schematic sectional view in FIG. 13. In FIG. 13, unessential layers such as an electrode contact layer are omitted.
A semiconductor laser relevant to the present invention is based on a waveguide structure having again. That is, an active layer 2 is formed on an n-type cladding layer 1 (including a substrate), and a p-type cladding layer 3 is grown on this active layer 2, thereby forming a layered structure in which the active layer 2 is sandwiched. This structure is generally widely known as a “double hetero structure”. An electric current is supplied into the shape of a stripe to the p-n junction of this double hetero structure via electrodes 4 and 5, thereby generating a gain. The active layer 2 has a waveguide function because it has a refractive index higher than those of the upper and lower layers 3 and 1.
To form a cavity for performing optical feedback, two end faces 10 and 11 are formed into mirror end faces by cleavage. These cleavage surfaces are laser light output ends. A high-reflectivity coating and an anti-reflectivity coating are formed on these end faces to control the Q value representing the performance of a cavity. That is, the threshold current, slope efficiency, and the like can be controlled by these optical coatings.
As described above, the semiconductor laser relevant to the present invention is not complete as a device unless cleavage end faces are formed. Hence, on-wafer processes and evaluations are difficult to perform, resulting in very low productivity.
A VCSEL (Vertical Cavity Surface Emitting Laser) is an example of semiconductor lasers using no cleavage end faces.
FIG. 14 conceptually shows the structure of the VCSEL. The VCSEL has a structure in which a thin gain medium (active layer) 2 is sandwiched between high-reflectivity multilayered films (DBRS: Distributed Bragg Reflectors) 12 and 13. Like a waveguide laser, the active layer 2 is sandwiched between cladding layers 1 and 3 to form a double hetero structure. In this VCSEL, no cleavage surfaces are necessary, but the small volume of the active layer 2 having a gain increases the current density. Therefore, gain saturation and heat generation make high-output operations difficult. This small volume of the active region is suited to low-threshold operations. However, high-output operations and high-temperature operations are more demanded in actual laser applications.
Noda also proposed a surface emitting laser using a photonic crystal in the abovementioned reference. This laser is shown in FIG. 15. The laser simply utilizes a high-reflection function of a photonic crystal W2. That is, the laser shown in FIG. 15 is fabricated by fusing a wafer W1 having cladding layers 1 and 3 and an active layer 2, and the photonic crystal W2. With this arrangement, a light emission area is large, so high output can be expected. However, the laser requires a lens to focus a beam to one point. A small beam spot is an essential characteristic in applications such as coupling to an optical fiber and read/write to a DVD (Digital Versatile Disk).
At present, forming an active layer in a photonic crystal and injecting an electric current into it encounter difficulties in fabrication. Therefore, a wafer having an active layer and a photonic crystal are separately fabricated and bonded by the fusing technology at a later time. That is, since there is no convenient method of readily integrating a gain region and a photonic crystal, a gain region must be formed outside a photonic crystal.
When the problems described in detail above of the devices relevant to the present invention are taken into consideration, the most expected laser has the following features.
(1) Surface light emission
(2) High output
(3) High-temperature operation (stably operable over a broad temperature range)
(4) Fine spot, narrow beam
(5) Relatively simple fabrication method
The present invention has been made in consideration of the above features. That is, it is the second object of the present invention to provide an optical device which overcomes the conventional technical problems by a unique arrangement based on a new idea and has desired characteristic features as described above, and to provide an optical module and optical system incorporating the optical device.